It doesn't seem to matter what I do with the slider. The card I select always ends up on top of my deck now. I'm pretty sure this used to be implemented correctly. Maybe something got messed up in preparation for Stash?
Edit: Actually it doesn't topdeck all the time, but the log always says that it is. I think it sometimes topdecks when I didn't tell it to, but am no longer sure.
I can confirm that the log always says that card XXX was "topdecked". But in a few experiments, with putting the card on top, bottom or in between or using 2 Secret Passages, I always drew the cards later where I put them.